explain compressors to me and why I "need" one
 

??

you might not need one.
start here

What he said... and, well, a compressor can be used as an effect or to even out the overall levels of your bass. Ever though your volume seems louder when you hit your low B or your low E then the other strings, however you play? Ever noticed a difference of volume between techniques? These are things a compressor can help you with... among others.

You don't necessarily NEED one. If you use any distortion or overdrive on your main tone you have a form of compression going on right there. If you ever record in a studio they'll add it. You can live without it playing live, but it's definitely helpful for evening out your volume.

More particularly, start with this article: http://www.ovnilab.com/articles/necessary.shtml

you shouldn't need a compressor if your playing style is consistent and your amp is eq'd properly, slap/pop guys excluded.

Sounds like I don't need one. Rock

It's not as simple as all that, but for the most part, you probably don't. I generally prefer not to use comps, but that's mostly because after trying a bunch of them I don't want ONE comp for everything, I want one for this and a different one for that. Since I'm so cheap, and none seem to provide me with any sort of holy grail of tone, I get none :).

If you have a few dimes to spare and are missing something consistent and identifiable from your tone that EQ can't seem to fix, you should poke your nose into a few stores that sell some good ones and see if a comp works for you. If you don't have that problem, you're probably good.